India plans to use fire trucks to spray water over parts of its capital to combat toxic smog and dust that has triggered a pollution emergency, with conditions expected to worsen over the weekend.

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Illegal crop burning in farm states surrounding New Delhi, vehicle exhausts and swirling construction dust have contributed to what has become an annual crisis.

Authorities will use the fire trucks in areas with high concentrations of toxic dust, said Ritesh Kumar Singh, an environment ministry official, after a meeting of civil servants from the city government and four neighbouring states.

Another ministry official who did not wish to be identified said water cannon - usually used by police for riot control - would also be employed.

"Sprinkling water is the only way to bring down the dangerous pollution levels," said Shruti Bhardwaj, an environmental official charged with monitoring air quality.

The thick blanket of grey air and pollutants has enveloped Delhi for four days. A US embassy measure of tiny PM 2.5 particles, showed a reading of 523 at nine a.m. on Friday. The upper limit of "good" air is just 50.

PM 2.5 is about 30 times finer than a human hair. The particles can be inhaled deep into the lungs, causing heart attacks, strokes, cancer and respiratory diseases.

Delhi's air has been consistently in the "hazardous" zone for days, despite measures such as a halt to construction and restrictions on car use including raising parking charges fourfold and a licence plate "odd-even" rule on alternate days. Commercial trucks are banned from the city unless they aretransporting essential commodities.

Air purifiers and masks

Chief Minister Arvind Kejriwal, the leader of Delhi, has called the capital a "gas chamber". Affluent residents and large companies have been placing bulk orders for air purifiers and masks.

"I have installed six air purifiers in my home and closed all the windows, but my eyes are still watering," said Ranveer Singhal, a commodity trader living in a leafy neighbourhood of the city.

Organisers of music festivals and open air parties cancelled events after the Central Pollution Control Board said air quality could deteriorate further during the weekend.

Pronab Sarkar, president of the Indian Association of Tour Operators, said air pollution had also hit tourism.

The National Green Tribunal has directed the Delhi government and neighbouring states to stop farmers from burning crop residue, but many have no choice, unable to afford machinery to clear fields instead.

The federal government said the states of Punjab and Rajasthan had failed to use funds allocated to them to help them in crop residue management and farm mechanisation.

Punjab, known as India's grain bowl, disputed the claims. A state government official said the funds were specifically meant for farm mechanisation, not for stopping the burning of straw.